Hydrous lithosphere and diffuse crustal accretion and tectonics in the southern Mariana margin: a possible analog for subduction zone infancy and ophiolites

Abstract:

The mode of extension and crustal accretion may vary significantly during subduction margin evolution. Mantle water content likely has a strongly influence on this evolution as it strongly affects the mantle solidus and rheology. Effects of mantle hydration on volcanism and tectonics were examined in the southern Mariana margin in 2012 on a R/V Thompson cruise. The southern Mariana margin is actively rifting sub-parallel to the trench forming new crust and lithosphere directly above the de-watering slab (see Ribeiro et al. session T011). Shallow seismicity shows broadly distributed active deformation in the upper plate. Shallow-towed and near-bottom sidescan sonar data map a highly faulted terrain with rotated crustal blocks and distributed volcanic emplacements. The near-bottom sidescan sonar data also image an apparent corrugated core complex structure, the first such described from a convergent margin setting, indicating low-angle normal faulting during the extension. Water content in sampled volcanics is ~2 %, approaching that of the volcanic arc itself. Volcanic rocks from the eastern margin are mostly ~2-4 m.y. old, but younger basaltic volcaniclastics were recovered farther west suggesting that active volcanism may continue. We hypothesize that the broadly distributed volcanism and tectonic activity is due to high mantle water content that weakens the margin lithosphere. Continual water addition from the subducting slab inhibits melting-related dehydration and strengthening as has been proposed for lithosphere formed at mid-ocean ridges. A consequence of a broadening zone of rifting is that extension-related mantle upwelling rates will decrease with time. Surface cooling will thus progressively depress the mantle solidus, perhaps explaining the paucity of current observed volcanism at the margin. The volcano-tectonic processes active today in the southern Mariana margin may be modern analogs of those inferred at subduction zone infancy where broadly distributed contemporaneous extension and volcanism above the initially subducting and de-watering slab have been proposed. If so, and if ophiolites are relicts of early crust formed at subduction settings, they may reflect crustal accretion processes that differ significantly from those at mid-ocean ridges.